Low tension ignition system



w. B. SMITS LOW TENSION IGNITION SYSTEM Filed Oct. 19, 1954 INVENTORATTORNEY United States Patent Ofiice 2,733,369 Patented Jan. 31, 19562,733,369 LOW TENSION IGNITION SYSTEM Wytze Beye Smits, Voorburg,Netherlands, assignor to Smitsvonk N. V., Leidschendam, Netherlands, aresearch laboratory Application October 19, 1954, Serial No. 463,249 8Claims. (Cl. 313-131) This invention relates to a low tension surfacedischarge spark plug and more particularly to the construction of thesparking element for such a plug. The low tension ignition system withwhich the spark plug according to the invention is to be used isdescribed in my Patent No. 2,506,472; and is the type in which acondenser is periodically charged from a voltage source and thendischarged across the electrodes of a surface discharge spark plug witha hot intense spark. A known low tension surface discharge spark plug isdescribed in my Patent No. 2,625,922 and consists primarily ofv twoconcentric circular electrodes, between which is disposed a separatingbody of insulating or semi-conducting material. The hot spark from thecondenser discharge burns across the surface of the separating body atthe point of least resistance between the electrodes. This is afundamental feature of spark plugs of this type and one which must begiven primary consideration in the design of such spark plugs.

A second fundamental aspect of these spark plugs is that their lifedepends upon a uniform burning away of the separating body. If the sparkacross the separating body was always at one point, and a separatingbody was burned away at that point deep in between the electrodes, thelife of the plug would be extremely short. On the other hand, if thespark occurred at varying locations around the circle between theelectrodes, the separating body would be burned uniformly and graduallyaway, and the life of theplug would be greatly extended. v

The construction shown in Patent No. 2,625,922 is designed to permit theuniform burning away of the separating body. Whilethe arrangement showntherein has proved highly satisfactory, it has'the disadvantage that theelectrodes must be precisely positioned with respect to each other sinceany eccentricity in alignment will result in a sparking occurring onlyat one point (the point of least resistance). The manufacture of thesespark plugs is precision work and calls for the utmost accuracy becauseof the requirement for precise alignment of the electrodes. The sparkplug as shown in that patent also has the further disadvantage that theplug has a restricted life since the insulating material between theelectrodes may be burned away inwardly only to a given depth. If burningtakes place beyond that depth, it will become difficult for the spark tocontact a combustion mixture and ignition will be unsatisfactory.

Upon wear, the spark plugs are dependent to a considerable extent on thevoltage applied between the electrodes; and with a combustion engine inwhich a good deal of oil deposits on the plugs there is always a risk ofthe spark burning inwardly only at one place rather than uniformlyaround the circle of the separating body.

It is the object of the present invention to eliminate the disadvantagesconsidered above and to provide a spark plug which can be manufacturedvery inexpensively.

It is a further object of thejinvention to provide a spark plug having auseful life which can, practically speaking, be adjusted to any desiredvalue.

.It is a further object of the invention to provide a for known sparkplugs. a

The disadvantages of the known spark plugs are eliminated byconstructing a spark plug having a large ing material, the sparkwillalways be exposedto the sparking surface, as compared with the abovedescribed the existing spark plugs.

As will be demonstrated, the spark plug according trodes during itswhole life, since the resistance between can be controlled. Tounderstand this, it .must be appreciated that the spark will occur atthe point of least resistance as mentioned above. If the thickness of:the insulating layer continually increases, the sparking willcontinually take place where the insulating layer .is

which the insulating material grows thicker. An0ther advantage in havinga very thin outer electrode is that the outer electrode together withthe .insulating mass will gradually be burned away while the otherelectrode remains substantially intact. Thus, good heat discharge fromthe core of the. spark plug is ensured.

According to one aspect of the invention, the thicker electrode mayconsist of a cylindrical metal body with a substantially semi-sphericalend. The enamel layer is applied to the thicker electrode from above asby pouring or dripping onto the tip of the semi-spherical end. Thisdripping process will cause the enamel to form a thicker layer at thelower end (at the base of the plug) than at the top of the semi-sphere.

The semi-sphere with its enamel layer is subsequently fired in order tobake the enamel layer.

After firing, a thin metal layer is applied to the enamel Further, sincethe area of the y These simple steps are all which are required to com--plete the formation of the active parts of the spark plug. In order toobtain the initial sparking point, a small portion of the thin metallayer (the outer electrode) and of the enamel layer is removed at thetop of the semi-sphere where the enamel layer is thinnest. It is at thispoint that the plug will first start sparking.

During continued operation, the whole of the spherical thin metalsurface will be burned away gradually. The spark will always be wellexposed to the combustion mixture, because the burning away of the outerelectrode always prevents a burning in between the two electrodes. 7

In order to provide an even more effective control of the burning awayof the electrode, before the application of the enamel layer, thespherical electrode may be provided with a number of shallow grooves. Inthese grooves, the insulating layer will be slightly thicker than atother parts of the electrode assembly so that the burning away will becontrolled in a given direction in segments.

Further advantages of the invention as well as those described abovewill become more readily apparent from the following detaileddescription of the invention taken in conjunction with the drawings inwhich:

Fig. l is an elevational view, partly in section, of the spark plugaccording to the invention,

Figs. 2, 3, 4 and 5 are cross-sectional views through the end portion ofthe electrode assembly and showing alternative embodiments of saidassembly, and

Fig. 6 is an end elevation of still a further embodiment of theelectrode assembly.

In Fig. 1 the numeral 1 indicates the plug body with screwthread 2,insulator 3, and central terminal 4. Secured in the screwthreadedpartwith the aid of an insulating mass 6 is the central pin 5, and this pinis connected with the central terminal 4. Screwed onto the pin 5 withthe aid of screwthread 7 is a substantially semispherical part 8 ofmetal, which serves as the tension electrode of the sparking plug. Thiselectrode is covered with an enamel layer 9, which from the point 10downwards gradually becomes slightly thicker, and from'the point 11becomes thickest. Down to this point the sparking surface may beconsumed, since the spark could no longer come into contact with thegas, if the spark could burn in deeper down. On this enamel layer 9 hasbeen applied a metal layer 12, which is connected with the plug body andserves as the second electrode. At this point 10 a small portion of thethin metal layer 12 and of the enamel layer 9 has been removed, so thatthere the first sparking point is formed.

Figs. 2 and 3 illustrate how this removal can be effected. According toFig. 2 the semi-sphere 8 has a projecting tip 13, which is broken 01fupon completion of the sparking element.

It is also possible to make a notch 14 in the semisphere 8 before theapplication of the enamel layer (Fig. 3). After completion of thesparking element the outer electrode and the enamel layer are thenremoved in this place by a stroke on a pointed object. From the topdownwards the metal layer 12 and the enamel layer 9 will be graduallyburned away, and finally all the material will have been burned awaydown to just above the point 11, upon which the electrode assembly 8, 9,12 has to be renewed. Since the area of said element is very large andcan be increased according'to requirement, this sparking plug has a verylong life. The enamel layer at the point 11 being so thick that thesparking plug can no longer be caused to spark without considerablyincreasing the tension, it cannot burn it, which would have thedetrimental efiect referred to above.

In order to increase the area of the sparking element even further, itmay also be given the vaulted form of Fig. 4 or the corrugated form ofFig. 5.

In the construction according to Fig. 6, the semispherical electrode 8is formed with shallow grooves 15. When the enamel is formed on thesemispherical electrode, that insulating layer will be thicker atgrooves 15 than on the remainder of the surface of electrode 8. Thus,the outer electrode, which is subsequently applied, and the insulatingmaterial will burn away more readily at the areas other than the grooves15. In this manner, it is possible to regulate the direction in whichthe materials will be burned away from the beginning to the end of thelife of the electrode assembly.

Finally, one more stripor point-shaped layers 12a of conductive materialmay be provided in the insulating layer, as a result of which on the onehand, better sparking action'is obtained while on the other hand, theburning away will become very uniform.

In a general manner, while there has been disclosed in the abovedescription what is deemed to be the most practical and efiicientembodiments of the invention, it should be well understood that theinvention is not limited to such embodiments as there might be changesmade in the arrangement, disposition and form of the parts withoutdeparting from the principle of the present invention as comprehendedwithin the scope of the accompanying claims.

I claim:

1. In a low-tension surface discharge spark plug for internal combustionengines, an electrode assembly comprising, an inner electrode, a coatingof insulating material on said inner electrode, and a thin outerelectrode disposed on said insulating material.

2. A spark plug according to claim 1, in which said outer electrodesurrounds substantially the entire inner electrode. a

3. A spark plug according to claim 1, in which said inner electrodecomprises a cylindrical metal body terminating in a semi-sphere.

4. A spark plug according to claim 3, in which a small portion of thethin metal layer and the insulating material is removed in one place.

5. A spark plug according to claim 1, in which said insulating materialis enamel.

6. A spark plug according to claim 1, in which the surface of the innerelectrode extending beyond the plug body is at least partly corrugatedso as to increase the "active surface.

7. A spark plug according to claim 3, in which the semi-sphere isprovided longitudinally with a number of shallow grooves filled withinsulating material.

8. A spark plug according to claim 3 in which the insulating material isthinnest at the tip of the semisphere and gradually increases inthickness toward the end of the electrode remote from the semi-sphere.

No references cited.

